In recent years, the energy problem is gradually taken seriously, and thus kinds of new energy-saving lightings have been developed. Among them, the light-emitting diode (LED) has advantages of high luminous efficiency, less power consumption, mercury-free and long lifetime, which has become a promising lighting source in the next generation.
With the use of phosphor and LED chip, the yellow phosphor, YAG (Yttrium Aluminum Garnet, Y3Al5O12), is excited by the blue LED chip to emit the yellow light, and the blue light of the LED chip complements the yellow light of YAG-based phosphor to emit the white light.
The method for coating the phosphor on the LED comprises conformal coating and remote phosphor. The method of conformal coating is performed by directly coating the phosphor on the LED chip to form a phosphor layer. The method of conformal coating has the advantage of uniform thickness. But the light emitted from the phosphor layer is absorbed by the LED chip and the support substrate, and the overall luminous efficiency of the LED is reduced. In addition, because the phosphor layer is directly formed on the LED chip, the high temperature between 100° C. and 150° C. during the operation of the LED chip gradually degrades and affects the conversion efficiency of the phosphor layer.
Remote phosphor is developed to solve the problem of conformal coating described above. The phosphor layer of the light-emitting device with remote phosphor is apart from the LED chip to avoid the light emitted from the phosphor layer being directly absorbed by the LED chip. In the method of remote phosphor, the phosphor of the phosphor layer is less susceptible to degradation under the high temperature operation of the LED chip.
The phosphor absorbs the light emitted from the LED chip and is excited to emit a light having a wavelength different from that of the light emitted from the LED chip. But the light emitted from the phosphor emits towards all directions, including the inward direction, and that reduces the luminous efficiency of the light-emitting device.